In December I hit the $1000 a month barrier mainly with freelance writing and sponsored posts. I'm hoping to build on that and hit $1500 by the end of this year.
Looks like you're doing pretty great!
How much time do you think you put in every week in order to earn that? Is it comparable to a 40 hour a week "normal" job?
Keep working at it. My advice is to diversify the sources as much as you can. To do that start out with one ad network and optimize for it then see what space you have left for other methods. Don't overdo it though, spam will cost you visitors!
When my GPT(s) were online, I was making a good $2k-$3k per month. Since I sold those projects I've moved to eBay and now making nearly $1,000 in sales per day! It was a great switch but I sure do miss the GPT business.